The Frontline River (戦線となる川,Sensen to naru Kawa, also in British English translation River Frontline) is a flowing river within Mugenri. The river goes in a straight line, and it has some woods on both sides of it.

The river appears on stage 2 of the Brilliant (right) route of Brilliant Pagoda or Haze Castle. Taking the Neutral route (middle) from this stage will keep you over the river in later stages, but you can go different ways for new locations depending on which route you take. As such, if you go right onto the Brilliant route, you'll go to the Fantasy Seawall, but if you go left onto the haze route, you'll end up back on the Shrine Road.